The present invention relates to an imaging sensor, and more particularly to a contact image sensor using electrically switchable Bragg gratings. This application incorporates by reference in its entirety PCT Application Number: PCT/GB2011/000349 with International Filing Date: Nov. 3, 2011 entitled BIOMETRIC SENSOR by the present inventors.
A contact image sensor is an integrated module that comprises an illumination system, an optical imaging system and a light-sensing system—all within a single compact component. The object to be imaged is place in contact with a transparent outer surface (or platen) of the sensor. Well known applications of contact image sensors include document scanners, bar code readers and optical identification technology. Another field of application is in biometric sensors, where there is growing interest in automatic finger print detection. Fingerprints are a unique marker for a person, even an identical twin, allowing trained personal or software to detect differences between individuals. Fingerprinting using the traditional method of inking a finger and applying the inked finger to paper can be extremely time-consuming. Digital technology has advanced the art of fingerprinting by allowing images to be scanned and the image digitized and recorded in a manner that can be searched by computer. Problems can arise due to the quality of inked images. For example, applying too much or too little ink may result in blurred or vague images. Further, the process of scanning an inked image can be time-consuming. A better approach is to use “live scanning” in which the fingerprint is scanned directly from the subject's finger. More specifically, live scans are those procedures which capture fingerprint ridge detail in a manner which allows for the immediate processing of the fingerprint image with a computer. Examples of such fingerprinting systems are disclosed in Fishbine et al. (U.S. Pat. Nos. 4,811,414 and 4,933,976); Becker (U.S. Pat. No. 3,482,498); McMahon (U.S. Pat. No. 3,975,711); and Schiller (U.S. Pat. Nos. 4,544,267 and 4,322,163). A live scanner must be able to capture an image at a resolution of 500 dots per inch (dpi) or greater and have generally uniform gray shading across a platen scanning area. There is relevant prior art in the field of optical data processing in which optical waveguides and electro-optical switches are used to provide scanned illumination One prior art waveguide illuminator is disclosed in U.S. Pat. No. 4,765,703. This device is an electro-optic beam deflector for deflecting a light beam within a predetermined range of angle. It includes an array of channel waveguides and plural pairs of surface electrodes formed on the surface of a planar substrate of an electro-optic material such as single crystal LiNbO3.
While the fingerprinting systems disclosed in the foregoing patents are capable of providing optical or optical and mechanical fingerprint images, such systems are only suitable for use at a central location such as a police station. Such a system is clearly not ideal for law enforcement and security applications where there is the need to perform an immediate identity and background check on an individual while in the field. In general current contact image sensor technology tends to be bulky, low in resolution and unsuitable for operation in the field.
Thus there exists a need for a portable, high resolution, lightweight optical contact sensor for generating images in the field.